Pervaporation is an energy efficient alternative to distillation for removing volatile organic compounds (VOCs) from water, especially solvents from their dilute solutions in a fermentation broth. Polymeric, ceramic, and liquid membranes have been used in the pervaporation process for removing such solvents. Liquid membranes generally provide high solvent selectivity and are therefore an attractive choice for membrane material. However, liquid membranes have a tendency to degrade rapidly. The instability of liquid membranes presents a major problem and drawback with respect to their use. In particular, three significant problems arise because of the instability and degradation of liquid membranes. First, membrane selectivity significantly decreases, resulting in a permeation of water through the membrane. Second, the process stream, e.g., fermentation broth, becomes contaminated by the liquid membrane. Third, the liquid membrane can disappear over a period of time.
In typical pervaporation processes, feed liquid containing volatile species flows on one side of a membrane, while the permeate side is maintained under vacuum. The driving force for the process is the difference between the hypothetical species partial pressure in equilibrium with the feed liquid and the permeate side partial pressure in the vapor phase.
Fermentation processes can generate a variety of byproducts, such as acetone, ethanol, butanol, acetic acid, propionic acid, etc. These compounds have a variety of potential uses, e.g., such compounds can be used as solvents, fuels and/or chemical intermediates. However, the solvent concentrations in a fermentation broth are often quite low; around 1-2% [see, e.g., N. Qureshi et al., Acetone Butanol Ethanol (ABE) Recovery by Pervaporation Using Silicalite-Silicone Composite Membrane From Fed-Batch Reactor of Clostridium Acetobutylicum, J. Membrane Sci., 187 (2001) 93-102]. For these low solvent levels in a fermentation broth, distillation is not an economical process for solvent recovery from the solution. Pervaporation is a promising technique for the separation of solvents from solutions having a low concentration of solvents.
A variety of membranes have been used in pervaporation processes for dilute solutions. Examples include:                Polymeric membranes made of polydimethylsiloxane (PDMS) (and composites) [see, e.g., N. Qureshi et al., Acetone Butanol Ethanol (ABE) Recovery by Pervaporation Using Silicalite-Silicone Composite Membrane From Fed-Batch Reactor of Clostridium Acetobutylicum, J. Membrane Sci., 187 (2001) 93-1021; L. M. Vane, A Review of Pervaporation for Product Recovery From Biomass Fermentation Process, J. of Chemical Tech. and Biotech., 80 (2005) 603-629; Y. Mori et al., Ethanol Production From Starch in a Pervaporation Membrane Bioreactor Using Clostridium Thermohydrosulfuricum, Biotech. Bioeng., 36 (1990) 849-853; M. She et al., Effects of Concentration, Temperature, and Coupling on Pervaporation of Dilute Flavor Organics, J. Membrane Sci., 271 (2006) 16-28; and P. J. Hickey et al., The Effect if Process Parameters in the Pervaporation of Alcohols Through Organophilic Membranes. Seperation Sci. Tech., 27(7) (1992) 843-861]        Polymeric membranes made of polytetrafluoroethylene (PTFE) [see, e.g, L. M. Vane, A Review of Pervaporation for Product Recovery From Biomass Fermentation Process, J. of Chemical Tech. and Biotech., 80 (2005) 603-629; Y. Mori and T. Inaba, Ethanol Production From Starch in a Pervaporation Membrane Bioreactor Using Clostridium Thermohydrosulfuricum, Biotech. Bioeng., 36 (1990) 849-853; A. Ghofar et al., The Pervaporation Mechanism of Dilute Ethanol Solution by Hydrophobic Porous Membranes., Biochem. Eng. J., 18 (2004) 235-238; and D. L. Vrana et al., Pervaporation of Model Acetone-Butanol-Ethanol Fermentation Product Solutions Using Polytetrafluoroethylene Membranes, Seperation Sci. Tech., 28 (13&14) (1993) 2167-2178]        Polymeric membranes made of polyvinyl alcohol (PVA) (and its composites) [see, e.g., W. F. Guo et al., Pervaporation Study on the Dehydration of Aqueous Butanol Solutions: A Comparison of Flux vs. Permeance, Separation Factor vs. Selectivity, J. Membrane Sci. 245 (2004) 1999-210]        Ceramic membranes such as silicalite (zeolite) membranes, alumina or composite membranes [see, e.g., N. Qureshi et al., Acetone Butanol Ethanol (ABE) Recovery by Pervaporation Using Silicalite-Silicone Composite Membrane From Fed-Batch Reactor of Clostridium Acetobutylicum, J. Membrane Sci., 187 (2001) 93-102; H. Matsuda et al., Improvement of Ethanol Selectivity of Silicate Membrane in Pervaporation by Silicone Rubber Coating, J. Membrane Sci., 210 (2002) 433-437; T. Ikegami et al., Concentration of Fermented Ethanol by Pervaporation Using Silicalite Membranes Coated With Silicone Rubber, Desalination, 149 (2002) 49-54; T. Sano et al., Estimation of Dealumination Rate of ZSM-5 Zeolite by Adsorption of Water Vapor, Zeolites, 16 (1996) 258-264; and T. Sano et al., Separation of Acetic Acid-Water Mixtures by Pervaporation Through Silicalite Membrane, J. Membrane Sci., 123 (1997) 225-233]        Liquid membranes of oleyl alcohol, decyl alcohol, tricresylphophate, tri-n-butylphosphate, and the like [see, e.g., Y. Qin et al., Pervaporation Membrane that are Highly Selective for Acetic Acid Over Water, Ind. Eng. Chem. Res., 42 (2003) 582-595; M. A. Fahim et al., Extraction Equilibria of Acetic and Propionic Acids from Dilute Aqueous Solutions by Several Solvents, Separation Sci. and Tech., 27 (1992) 1809-1821; R. Wodzki et al., Propionic and Acetic Acid Pertraction Through a Multimembrane Hybrid System Containing TOPO or TBP, Separation/Purification Technology, 26 (2002) 207-220; and M. Matsumura et al., Separation of dilute aqueous butanol and acetone solutions by pervaporation through liquid membranes, Biotechnol. Bioeng. 30 (1987) 1991-1992].        
In general, the concentration of solvents in the permeate stream from polymeric membranes and ceramic membranes are lower compared with those from liquid membranes. The liquid membranes generally have a higher selectivity, making liquid membranes an area of significant interest for the recovery of dilute amounts of solvents from aqueous solutions.
With reference to the patent literature, U.S. Pat. No. 5,637,224 (the “'224 patent”) relates to the removal of volatile organic compounds from aqueous solutions using a hollow fiber contained liquid membrane. The '224 patent is directed to a system for transferring a vaporizable solute from a feed solution to an extractant liquid and then removing the solute from the extractant solution. Hollow fiber supported liquid membranes (SLM) and plasma polymerized non-porous silicone coatings are disclosed in the '224 patent. U.S. Pat. No. 5,933,515 (the “'515 patent”) discloses hollow fiber modules that rely on vacuum driven pervaporation with single fiber units for removal of solvents. In addition, the problem of liquid membrane stability is widely discussed in the art. Typically, the patent literature has been directed to methods for replenishing the liquid membrane as it degrades (see, e.g., U.S. Pat. Nos. 6,433,163, 4,973,434 and 6,096,217).
The literature also includes two (2) publications co-authored by the present inventor: (i) “Supported Liquid Membrane-Based Pervaporation for VOC Removal from Water,” published in the Industrial and Engineering Chemistry Research Journal in 2002, and (ii)“Novel Membrane and Device for Vacuum Membrane Distillation-Based Desalination Process,” published in the Journal of Membrane Science. The two publications are briefly summarized as follows:                Supported Liquid Membrane-Based Pervaporation for VOC Removal from Water: The publication discloses experimentation involving use of a liquid membrane supported by a plasma-polymerized non-porous silicone coated microporous hydrophobic polypropylene hollow fiber membrane for removal of VOCs from waste water streams. The disclosed liquid membranes are formed by mixing hexane with a pure dodecane supported liquid membrane. The mixture was immobilized in the support substrate and the hexane was removed by applying a vacuum to the shell side of the fibers, leaving a thin dodecane layer in the pores. In the reported experiments, the feed solution is passed through the hollow fibers, with the vapor drawn through the porous/non-porous layers to the exterior of the fibers.        Novel Membrane and Device for Vacuum Membrane Distillation-Based Desalination Process: The publication describes a plasma-polymerized ultrathin porous coating of fluorosilicone on a porous hydrophobic polypropylene hollow fiber membrane. The coating was used to prevent wetting and fouling of the pores in the hollow fiber membrane during a desalination process. In the disclosed desalination process, water vapor passes through a hydrophobic porous membrane, so as to yield a concentrated brine solution on one side and purified water on the other. In the context of desalination, a highly selective liquid membrane barrier is not needed as is the case for the removal and recovery of volatile organic acids from aqueous solutions.        
In the field of liquid membranes, various membrane materials have been disclosed. For example, U.S. Pat. No. 6,171,563 discloses a two step process for the removal and recovery of chromium from waste water using a trioctylamine (TOA) supported liquid membrane embedded in a microporous support in a liquid to liquid extraction technique.
Despite efforts to date, a need remains for apparatus, systems and methods that provide, inter alia, enhanced selectivity and stability in a pervaporation process. Moreover, a need remains for apparatus, systems and methods for use in removing solvents, e.g., VOCs, from a fermentation broth. These and other needs are satisfied by the disclosed apparatus, systems and methods.